1. Field of the Invention
The present invention relates to a magnetic storage device. For example, the present invention relates to a form and placement of parts in a memory cell.
2. Description of the Related Art
The most-widely used type of a memory cell of a magnetic random access memory (MRAM) is 1T1R type which includes one magnetoresistance effect element and one select transistor. Write current circuits are connected to both ends of two types of write lines for generating a magnetic field which is applied to each magnetoresistance effect element. An electric current flows from the write current circuit at one end toward the write current circuit at the other end. Providing the write current circuits between adjacent memory cell arrays allows the write current and control signals to be shared by more than one memory cell arrays, which can realize simpler circuitry.
In the 1T1R type memory cell, the magnetoresistance effect element is electrically connected to the select transistor through a bottom electrode and a plug. Since the magnetoresistance effect element needs to lie between the two type write lines, the plug is placed away from a cross point of the write lines. The bottom electrode has a flat shape which is different from that of the magnetoresistance effect element in order to be connected to the plug. In addition, the bottom electrode may have a flat shape, such as an L-shaped form, in order to facilitate integration of the memory cell.
All the memory cells desirably operate in the same manner in order to secure margins of operation. In order to achieve such request, each part, such as an electrode, the magnetoresistance effect element, and interconnect, is made to have a same physical form to unify the characteristics of each memory cell (refer to Dietmar Gogl et al., “A 16-Mb MRAM Featuring Bootstrapped Write Drivers”, IEEE journal of Solid-state Circuits, April 2005, vol. 40 pp. 902).
The IEEE journal reference does not refer to a write magnetic field. However, in order to make the characteristics of each memory cell homogeneous, even a direction, size and so forth of the write magnetic field are desired to be the same between each memory cell. In order to achieve this, a relative positional relationship of the electrode, the magnetoresistance effect element (especially a free layer), the interconnect, etc., is required to be the same in each memory cell.
However, if the write current circuit which can only supply a write electric current is provided between adjacent two memory cell arrays, a direction of the magnetic field applied to the memory cell is different between the memory cell of one of the memory cell array and the memory cell in the other memory cell array. Therefore, the magnetic field is applied differently to memory cells.
In addition, a bottom electrode form is known to influence the write magnetic field. One of the reasons for this is that the write magnetic field and the bottom electrode are coupled electromagnetically to influence the write magnetic field, and the coupling depends on the form of the bottom electrode. In particular, if the flat form of the bottom electrode is asymmetrical, the magnetic field is applied to the memory cells largely differently.
Further, due to reasons such as convenience in manufacturing processes, a fixed layer may have a form same as the bottom electrode. Magnetization of the fixed layer forms a leakage magnetic field in a direction corresponding to the direction thereof, and changes the write magnetic field. Then, the leakage magnetic field is influenced by a form of the fixed layer, that is, the form of the bottom electrode. Due to such reasons form of the bottom electrode creates non-uniformity in the way of applying the magnetic field to the memory cells.
As described above, depending on a combination of differences of the physical form and the relative positional relationship of each part such as the electrode, the magnetoresistance effect element, and the interconnect, the characteristics of the magnetic field applied to the memory cell are different. As a result thereof, the margins of operation are narrow.
FIG. 8 of Jpn. Pat. KOKAI Appln. Publication No. 2005-236177 discloses that two MRAM macros RMCA and RMCB are arranged in symmetry about an imaginary axis parallel to the magnetization hard axis of a magnetoresistance effect element VR. In addition, FIG. 27 discloses that two MRAM macros RMCJ and RMCK are arranged in symmetry about an imaginary axis parallel to the magnetization easy axis of the magnetoresistance effect element VR. According to the disclosure, the arrangements can attain consistency between write data and read data. Note that in the publication the directions of the write magnetic fields are all the same regardless of a configuration of the MRAM macro for writing the same logic (“0” or “1”) data.